Upload
gil-travish
View
609
Download
0
Tags:
Embed Size (px)
Citation preview
What could you do with a particle accelerator on-a-chip?Advances in Dielectric Laser Accelerators and Prospects for Applications
Gil TravishUCLA Department of Physics & AstronomyParticle Beam Physics Laboratory&Radius Health
Presented at Varian Medical on 12SEP11
> 10 million new cancer cases each year> 50% of cases require radiation therapy
Current treatment issues:Immediate and long term side effects
Targeting of tumorsHandling and containment of radiation
High capital costs
what happens if we shrink this all down by
1000 ?
Our long term goal is to develop a mm-scale, laser-powered, disposable, relativistic particle source
>10m
<10mm
x1000
Large Application Space:
Industrial
• Petroleum Exploration
• Non-Destructive Testing (NDT)
• X-ray Photolithography
Medical
• Cardiology
• Veterinary
• Medical Imaging
Defense
• Package screening
• Stand-off detection
The conceptual design encapsulates the accelerator as a fiber optic tip
Fiber
(to
lase
r)
End
osco
pe e
nd
Enc
losu
re
MAP
Tum
or
Par
ticles
MAP technology irradiates the tumor and avoids healthy tissue
external beam
minimally invasive
vs
Bringing the source closer to the site allows electrons to deposit energy over a tumor depth
Electron Range and Stopping Power in Soft Tissue
soft tissue density = 1 g/cm3
1-6 MeV electrons have ~1-3 cm rangeminimal stopping power = minimal surrounding tissue damage
Reducing a therapy machine’s size may alter the economics of oncology treatment
BigHeavyExpensive ($3-5M)Microwave powered
Microchip sizedEndoscope tipDisposableLaser powered ($200k)
EBRT Machine MAP
What
magic scalingare we using to make this happen?
Breakdown limits scale favorably with wavelength and dielectric materials support high fields
10-15
10-13
10-11
10-9
10-7
10-5
10-3
10-1
100
102
104
106
108
1010
1012
1014
Pu
lse
Le
ng
th [
s]
Frequency [Hz]
GHz THz IR-VISfs
ps
ns
us
Conventional RF
DWA
L
A
S
E
R
Du (1996)~GV/m
T-481
Breakdown LimitsConventional Structure
Eacc ~ Prf/λ
in metals...
Of available power sources at wavelengths shorter than microwaves, lasers are the most capable
lack of sources, materials and fabrication technology force us to make a leap from Microwave to Optical
300MHz 3GHz 30GHz 300GHz 3THz 30THz 300THz
Visible and UV…Radio Frequency Terahertz
Mobile phones
Satellite TV Medical & industrial lasers
Terahertz gapGround to satellite
The choice of accelerator technology impacts the size and nature of the beam produced...
RF Optical
Gradient
Energy gain per period
Repetition Rate
Charge per Bunch
Bunch Length
10-100 MeV/m 1-10 GeV/m
1 MeV 1 keV
100 Hz 10-100 MHz
0.1 - 1+ nC 0.01-1 pC
1-100 ps 1-100 fs
key: charge and time scale; not gradient
McGuinness
Eacc ~ Prf/λBreakdown limits metal:
Optical structures naturally have sub-fs time structures and favor high rep. rate operation
3.3 fs charge capture< 1 fs
//
Fill Time ~ 1 ps Fill Time ~ 1-5 ps
Optical Cycles
Laser Pulse
femtosec
picosec
//Emitter Pulse
nanosecEmission Time ~1 ns
100-1000 ns(1-10 MHz)
Macropulse
Micropulse
Laser Cycle
A variety of optical-scale dielectric structures are under consideration
MAP Logpile Grating
~2.5 GV/m
PBG-fiber-based structures afford large apertures and length-scalability
Planar structures offer beam dynamics advantages as well as ease of coupling power
The MAP structure consists of a diffractive optic coupling structure and a partial reflector resonator
laser light
Gun
sub-relativistic relativistic
ShortBraggStack
Coupler
TallBraggStack
VacuumGap
electron beam
MicroAcceleratorPlatform
The three functional parts of the MAP—gun, low energy section and relativistic section—form one system
➊ Integrated “gun”
➌ Relativistic structure
➋ Low beta structure
Field Emitter
Bragg FieldDeflector
relativistic structure ➌
The design of the relativistic structure is mature and includes realistic material properties.
Ez = E0 cos(! z c) !
laser
gap (1 optical wavelength)
cot kz ! "1 b " a( )#
$%& = kza ! "1 ! !
For gap a anddielectric b-a
idealizedresonance:
Tuning: control “matching” layer (b-a).
The MAP is a moderate-to-low Q structure which matches well with existing laser technology
Parameter Value
Laser wavelength 800nm
Cell length 800nm
Effective gradient 1.5 GeV/m
Quality factor Q 800
Effective shunt impedance R 2000 ohms
Effective shunt impedance per unit 2.0833 ohms
R/Q 2.5 ohms
R/Q per unit 0.0026 ohms
Transit factor 0.86
Stored energy 0.9 mJ
Power dissipation <1% (0.75 MW)
Fill time 0.5 ps
Laser intensity 100 MW
Laser pulse length 1.8ps
Energy gain per unit cell ~2.5keV
Simulations show energy spectrum clean-up; and, include acceleration, beam dynamics and material properties.
Resonant Fields (@ t = 7 ps)Incident laser
y(m)
x(m)
Ex (V/m)
t(s)
t(s)
Ex (V/m)
Ex (V/m)
Input laser sourcecan correspond to actual Ti:Al2O3 laser
Energy Distributions Energy Gain
low energy structure ➋
at 1 GeV/m, each period only produces 1KeV1000 periods only yields 1 MeV
Creating a sub-relativistic MAP is hard:the coupling and periodicity are one and the same
tapered structure
periodicity variation
two-color operation
DTL-like Solutions
periodicity skipping
Thick Glass Substrate
!!/"
!! 2!
laser light
β
z (mm)
0.3
1
0.65
0 0.5 1
rapid change in velocity
The accelerating field may die off before the
particle fullly dephases
2.4 µm = 10βλ
β=0.3e-beam
λ=800 nm incident laser
Electrons’ energy gains outweigh losses, resulting in net acceleration
We have a viable design for the low energy accelerating structure.
0
-0.5
+1.0
0 1 2 µm
100 keV energy gain over~250 microns
0.4 GeV/m Accelerating Gradient
We have convincing evidence of energy gain for low-beta electrons in simulations
25 ke
V
125 k
eV
25 ke
V
125 k
eV
deph
asing
&
wall lo
sses
particle source (gun) ➊
A micro patterned Ferroelectric crystalcan be used as an integrated gun
Metalized, Grounded Scintilator: Electron (Camera) & X-ray Detection
Metalized LiNbO3 Crystal:Micro-Emitter(s) in Center
Grounded Heat Dissipater
TEC Heater
Additional Copper Heat Sink
Test Rig for Field-Enhanced Emissionfrom LiNbO3 Crystals
Before After Brightness - Background
Field overlay plot showing the magnitude of the electric field
An isolation region is able to control the fields inside the emitting region
FE Control Scheme
Integration of a gun into the low beta structure completes the source.
cont
rol
field
isol
ator
emitt
er
With field isolation, we can use any emitter
These pyroelectric emitters are pretty nifty so we decided to try to use them on their own.
We have spun off this pyroelectric + emitter technology to produce a flat panel x-ray source
!"#$%&'%()*'%*+,*
-!%."')/),'&+(')(,#$$',
'&#**#+%)#%.#0!(+,
1!2',3)"'4'"
'&#**#+%)!,'!
MAX: Microemitter Array X-rays
...with this.
Tube alone = US$ 25k (CRP) Target Price to OEM = US$20k
By replacing this …
Replacing vacuum tubes reduces manufacturing and maintenance costs.
We have generated arrays of prototype scale & small area images
Shaped Copper Mask
Cockroach
Where are we at with dielectric laser accelerators?
Thousand period structures—mm long and ~1 MeV gain—are now being produced.
Full scale coupler DBR
96.2nm
96.2nm
92.4nm
130.8nm
134.6nm
Structure Dimension: 300nmX250μmX1000
287.6nm
+
DBR+Coupler
Fusion bonding of the top and bottom slabs seems to be the most promising integration method.
1. DBR deposition on optical substrate 2. Patterned SiO2 deposition for gap support
4. Build matching layers and DBR
3. Release layer Cr and coupling layer fabricated on Si substrate
5. Bond the two pieces together by fusion bonding
+
We have begun a ß=1 MAP beam de/acceleration experiment at SLAC’s E163
E-163 PMQs
structure causes energy loss (dE/dx) to misaligned particles
no structure at IP
IPspectrometer
J. England
1’’ 1mm250 µm
800 nm
Glass dummy structure
1 mm
Slot in dummy structure (not to scale)
Aluminum holder for glass structure
e-beam
A “dummy structure” and mount was designed for beam transmission studies.
Bunches from NLCTA Beamline
Spot size = 96 x 83 µm2
εx = 43 µm-radεy = 24 µm-rad
electrons that lost energy while traveling through glass
electrons that made it through slot
Spectrometer Image (higher energy to the left)
‣Theoretically, we expect peaks to be separated by 0.5 MeV
‣With calibration of 1.776 KeV/pixel, we find separation of 0.337 MeV
For the first time, beam was transmitted through the optical-scale structure!
Data analysis is ongoing
Just one more thing...
an all optical light source
A MAP-based undulator structure has been designed
For E=3 GV/m,Beqv=10 Tesla
Undulator Period = Laser Phase Flip
E-field
…………
λu >> λlaser
waveplate
It is possible to have an all-laser-powered x-ray source using optical accelerator structures...
... but compromises must be made
low energy+
optical undulator=
QFEL
high energy+
conventional undulator=
FEL but long
predictionA particle accelerator “on a chip”, capable of
producing intense pulses of relativistic electrons and x-rays will be widely available
in 10 5 years
Funding:NNSADTRAUCLADOE
Acknowledgments
Team:Rodney YoderJianyun Zhou (Postdoc - Fabrication)Josh McNeur (Grad - Simulations)Esperanza Arab (Staff - Engineering)Several past and present students...